This invention relates to thick film silicon growth techniques.
An important photovoltaic device useful in the direct conversion of solar energy into electrical power is the silicon solar cell, but its use on a large commercial scale has not been feasible due to the lack of an industrially feasible process for the growth of single crystal silicon directly and continuously from a melt. A method which has been most successful is an edge-defined, film-fed growth (EFG) process by which crystals may be grown with their thickness and width defined and controlled by the outside dimensions of a die.
The EFG process employs two spaced apart sheets of suitable material partially immersed in a silicon melt. The silicon melt wets the sheets and rises to the top of the sheets by capillary action. A single-crystal silicon seed is brought into contact with the melt to initiate crystallization. After proper adjustment of the melt temperature, crystal withdrawal rate, and crystallization spread across the top of the sheets through their full width, a ribbon of crystal can be grown of indefinite length. This spreading across the top of the sheets is halted by the sharp change in the effective contact angle at the outer perimeter of the ends of the sheets. The result is a thick film silicon crystal ribbon the thickness and width of which are defined by the perimeter of the ends of the sheets.
To better control the thickness of the silicon crystal ribbon, it has been suggested that the sheets be beveled to provide two sharp ends, i.e., to provide only sharp edges wetted by the silicon melt. The perimeter of the ends of the sheets is thus defined by the two parallel edges at the upper ends of the beveled sheets. In other words, by reducing the outer surfaces wetted by the silicon melt to sharp edges, the silicon crystal formed is virtually restricted in thickness to the space between the sheets.
A problem with the EFG process is the random occurrence of twinning and other effects as the silicon ribbon is grown. When twinning occurs in a direction parallel to the length of the ribbon, the ribbon is better for the manufacture of solar cells, but when twinning occurs randomly, and in an uncontrolled manner so that there are different orientations of the twinning defects along the length of the ribbon, that portion of the ribbon is poorer and is discarded.
The defect called twinning is evidenced by replication of crystal planes in mirror image formations. As much as 5 feet of ribbon may be crystallized before twinning stabilizes in the preferred direction parallel to the length of the ribbon and normal to the crystallographic plane. Since ribbon growth is a slow process, such loss of ribbon is costly because of the substantial loss of process time and the unnecessary wear on the walls of the die surrounding the opening imposed by the passage of the hot, corrosive liquid silicon. The latter affects both configuration and dimension of the opening in time so that the die must be replaced. A ribbon growing technique which would reduce or eliminate the uncontrolled orientation of the twinning effect would be of considerable advantage.